OFDM technology uses an approach based on the division of a wireless communication channel into several sub-channels which can be used by multiple mobile stations at the same time. These sub-channels and the mobile stations are often subject to interference coming from neighbouring cells which are adjacent to the current cell to which a mobile station is connected. This occurs because neighbouring base stations can use the same frequency blocks that the ones used by the base station to which the mobile station is connected.
Thus, methods were proposed for adapting the output power of the base stations to a value which enables mobile stations at the cell edge to still transmit and receive data at quite a good rate while not creating too much interference in neighbouring cells. One of these methods, described below, has been proposed to reduce interference caused by neighbouring base stations using the same frequency blocks and thereby help to increase mobile station throughput and overall network capacity.
For distant mobile stations, (i.e., mobile stations at the edge of a cell), a base station has to use more transmission power in order to reach them. Mobile stations close to the base station require much less transmission power to receive the signal. Because known mobile stations only transmit and receive on some but not all sub-channels of the frequency band, transmission power of sub-channels used by mobile stations close to a base station can be lower than the transmission power of sub-channels used by mobile stations at the cell edge. In practice, the reduced transmission power for sub-channels used by mobile stations close to a base station creates less interference for mobile stations close to other base stations.
To further minimize interference of high power sub-channels for clients of neighbouring base stations, the cells are also organized in a way that two adjacent cells do not use the same high power sub-channels: the cells are thus divided in sectors (basically 3). As such, cell edge mobile stations can be scheduled on the high power tones that are not used or are used with lower transmit power by the neighbouring sectors of the neighbouring cells. This method is known as fractional frequency re-use (“FFR”). All base stations use the same frequency band with different power level restriction on different sub-channels. Some frequencies are used by all the sectors and thus have a reuse factor of one, whereas, other frequencies may only be used by a third of the sectors and thus have a reuse factor of ⅓.
FIG. 1 is an example of a cellular network CN with 3 cells (C1, C2, C3), each of them implementing the FFR method. FIG. 1 shows the cell edge as the “white” area, where the fractional frequency-reuse is more beneficial (only one frequency is used, for example F1): the “payment” of lost resources (for example F2+F3 in cell C1) is worth the reduction of the inter cell interference; The “grey” area utilizes the full bandwidth (i.e. all the frequencies F1+F2+F3), where the penalty of lower transmission rates is worth the excess bandwidth made available.
An obvious drawback of this FFR method is to reduce throughput capacity (just ⅓ of the normal capacity). Therefore, in order to overcome this drawback, FFR is used in conjunction with Full-PUSC on the same DL sub-frame, i.e. the data section of the DL sub-frame is divided into two independent zones (3 zones altogether, including the mandatory non STC ⅓ PUSC zone) as shown in FIG. 2 (which shows an example of a WIMAX TDD frame partitioned into zones on the DL: the partial reuse zone is the 1st and the full bandwidth reuse zone is the 2nd). PUSC is for “Partial use of sub-carriers”. PUSC method is for designating a method for creating sub-frequency channels, at the MAC level of the WiMAX standard. Beginning of each frame contains a UL-MAP and DL-MAP that indicate when and on what frequency the station can transmit (basic set of slots). These slots are placed in different sub-frequency channels, and when using a PUSC permutation, only a portion of the frequency band is used.
In other words, the common technique is to divide the data section of the DL sub-frame into two independent zones (altogether 3 zones including the mandatory non STC ⅓ PUSC zone).
Considering such a split of the DL sub-frame, the standard algorithm allocates mobile stations with high inter cell interference to the FFR zone and mobile stations with low inter cell interference to the full bandwidth zone. The standard algorithm calculates an index: the ratio between the value of allocating in the full reuse zone and the value of allocating in the fractional reuse zone. The ratio is compared to a threshold (typically 1) and mobile stations are allocated to the best zone, accordingly. The value is mainly an issue of the physical (PHY) layer.
In a TDD system, the fractional frequency reuse zone length must be shared by all the cells in the network and is therefore externally configured and can't be changed dynamically according to a specific cell's traffic pattern.
Considering the above observations, one of the main problems that might arise from the FFR concept is throughput reduction and scheduler inefficiency. Although mobile station assignment is dynamic over time, it doesn't take into account traffic request criteria, and thus it is expected that in many cases the inefficiency in the scheduling algorithm shall be quite high.
The inefficiency is evident when one of the zones (full or fractional reuse zone) is fully booked, while the other is not; DL packets that could have been transmitted during this frame are withheld since they were allocated to the full zone, and will not be transmitted in the partially empty zone.
Therefore, the algorithm must take into account the zone utilization when allocating mobile stations, in order to utilize both zones as much as possible. This is mainly an issue of MAC layer.